The nocturnal boundary layer transition from weakly to very stable. Part I: Observations

The nocturnal boundary-layer regime transition from weakly stable (strong wind) to very stable (weak wind) is analyzed using 10 levels of turbulence observations made at a 140 m micrometeorological mast near the southeastern Brazilian coast. The combination of synoptic and local flow favors the systematic occurrence of such a transition, typically 5 to 7 h after sunset. The regime transition is marked by decreases in temperature, wind speed, turbulent kinetic energy (TKE) and absolute heat flux. The decrease in temperature is often abrupt and the inflection point in the temperature series marks the regime transition. Absolute heat flux peaks before the transition during the weakly stable period, while temperature variance peaks near the transition. Composites from 36 cases when the cooling rate exceeded 2 °C/h are used to describe the vertical structure of the stable boundary layer (SBL) in both regimes. For these abrupt transitions, dimensionless variables that relate thermal and mechanical properties of the flow are compared as indicators of the SBL regime, and the gradient Richardson number is found to be better for that purpose. The absolute heat flux is shown to be proportional to the cube of the wind speed only in the strong wind limit of the very stable regime. Simulations of similar transitions using a second-order model are described in Part II.